Winchester 209 primers seal the best as they have the widest dimensions. Also what Bobcat said, use a rubber o-ring in the primer pocket. That will eliminate blow-back. Also you want to make sure and keep the flame channel clean on your breech plug.
I just traded for a Winchester Apesame as cva kodiak) on this forum a week or two ago. I had it out this week and it wouldn't ignite the powder at all. After tearing it completely apart, polishing the firing pin, chamfering and polishing the firing pin hole, checking springs, sear and everything else, I took it back out and same problem. I swapped in a new breech plug and bang!
All my CVA muzzys had serious blowback and occasional misfires until I started using Win 209 primers and using o-rings. Not a problem since I've done those things. Check out modern muzzleloader forums for more info. There is a chart for primer dimensions which is also helpful. Some people use stainless steel washers for a shim instead of o-rings.
The o-rings I'm using are 1/4 x 1/8 x 1/16 part# 780003 Faucet 1-1a; I think they are a Hillman product but I'm not sure. I just happened to have a package of them in my wallet so I can get more when I'm at the hardware store.
Good luck and let us know how it goes.